In mobile communications systems, the receiver obtains a distorted version of the transmit signal due to channel impairments such as multipath echoes and Doppler effects. The receiver needs to compensate the distortions of the channels by processing the received signal according to the knowledge of the channel distortions. In order to obtain the knowledge of the channel, the transmitter transmits training symbols—called pilots—which are known to the receiver. The receiver uses the channel knowledge at the pilots and extends this knowledge to other data symbols, to compensate the effect of the channel.
In current mobile communication systems such as LTE (Long Term Evolution), pilots are inserted at fixed time/frequency spacing (fixed density). Those spacing are chosen according to the worst expected channel fading in time and frequency. Fixed pilot spacing is only optimal when the channel is exactly at the worst expected channel. If for any reason, the channel is worse than the worst expected channel, then the reliability of the link is degraded. If the channel is better than the worst expected (which is the most common case), then the pilots act as an undesired overhead which could have been used for data transmission to boost the throughput instead.